Liars and Truth Tellers

You approach two magic talking doors. One door leads to the City of Truth, while the other door leads to the City of Liars. You do not know which door is which. You are able to ask only one question to determine which door is which. The door that leads to the City of Liars always speaks lies, while the door that leads to the City of Truth always speaks the truth. You want to go to the City of Truth. What question do you ask to determine which door leads to the City of Truth?

Answer

Answer #1: You ask a door - "If I were to ask the other door which door leads to the city of truth, what would he say?" and then you pick the opposite door of what he tells you.
Answer #2: Ask the door - "If I asked you which door leads to the city of truth before, which door would you have told me?" (negative and negative = positive, positive and positive = positive). and you choose the door that it directs you to.Hide